Paper reference standard, spectrum

Figure 1. Spectrum of revised paper reference standard 967.68.5 26 elements at 20 fig / cm2 density level vertical scale, 2,000 pulse counts full scale excitation schedule according to Table V.

Weigh out 7 mg. of infrared-quality powdered KBr into a 2-cm. micro-porcelain boat, and with a spatula lightly tamp into a small cake at one end. Add the concentrated eluate from the thin layer plate to the KBr in 2.0-/Jiter increments, allowing sufficient time (about 30 seconds) between additions for the solvent to evaporate. After all the sample has been evaporated onto the KBr, prepare the disc, and obtain the spectrum in the usual manner. It is advisable to record the spectra of a blank KBr pellet and a reference standard on the same paper for comparison. 

To provide the internal reference standard, TMS must be added to the sample solution. This substance has the formula (CH3)4Si. By universal convention, the chemical shifts of the protons in this substance are defined as 0.00 ppm. The spectrum should be shifted so that the TMS signal appears at this position on precalibrated paper. 

In this paper, we address two aspects of this general problem. First, we discuss the problem of frequency standards in the optical spectrum. (An analogue in the microwave region of the spectrum is the cesium beam frequency standard.) If one or a few of these reference frequencies can be accurately calibrated (perhaps by a frequency synthesis chain- -) then it may be possible to compare optical spectra to these standards. As an example of the precision that might be achieved, we discuss only optical standards based on stored ions. Second, we discuss the problem of frequency comparison of unknown frequencies to the standards. Here we primarily restrict discussion to generation of wideband frequency "combs". 

For infrared analysis, each standard quartz filter or redeposited sample filter is cut in half with surgical scissors and placed on the 30 or desired angle setting of the MIR ccessory base. The IR is adjusted to a scan setting of 750 cm and the reference beam attenuator is used to bring the transmittance level to 90-l(j)0%. The filter is then scanned in duplicate from 900 to 700 cm. pie spectrum is recorded yn absorbance chart paper. 

The term volatility is usually taken to refer to the speed at which a material evaporates. It is not an exactly defined property, and no universally accepted standards are laid down within the scientific literature. As implied above, the dry-down or evaporation behaviour of even an unsophisticated perfume on a simple solid substrate, such as a paper smelling strip, is complex. Some materials are so volatile that they are lost much more rapidly than other components (within minutes), while materials at the opposite end of the volatility spectrum may remain for a considerable time (weeks or months). However, there... 

If you are not using an FT-IR spectrometer, it is good laboratory practice to calibrate the experimentally obtained IR spectra against a standard that has absorption bands at known positions. Calibration is necessary because problems such as misalignment of the chart paper in the instrument or mechanical slippage may result in absorptions appearing at wavenumbers that are incorrect. Polystyrene (Fig. 8.10) is a commonly used standard and is typically available as a thin, transparent film in a cardboard holder that fits into the spectrometer. After the spectrum of the sample is obtained, the standard is inserted in place of the sample cell the reference cell, if used, is removed and at least one band of the standard is recorded on the spectrum of the sample. In the case of polystyrene, the absorption at 1601 cm is typically recorded, although other bands may be used. 

Once the reference p>eak of the standard is measured, the reported positions of the absorptions of the sample are adjusted accordingly. For example, if the 1601 cm peak of polystyrene appears at 1595 cm on the spectrum, the wavenumbers for the bands in the sample are corrected by adding 6 cm to the values recorded on the chart paper. Because the source of the error may vary over the range, it is not strictly correct to use the same correction factor throughout the spectrum, but for most purposes this is a satisfactory protocol. For more precise work, calibration peaks should be recorded at several different locations in the IR spectrum. 

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